The brain isoform of creatine kinase has been implicated in cellular transformation processes. Cyclocreatine, a creatine kinase substrate analog, was previously shown to be cytotoxic to a broad spectrum of solid tumors. We have synthesized, enzymatically characterized, and evaluated the antitumor activity of a series of substrate analogs of creatine kinase. Using in vitro assays, we demonstrate that several of these analogs are cytotoxic to the human ME-180 cervical carcinoma, the MCF-7 breast adenocarcinoma and the HT-29 colon adenocarcinoma cell lines at low mM concentrations. Analogs that were active in vitro delayed the growth of a subcutaneously implanted rat 13,762 mammary adenocarcinoma. Tumor growth delays of 6-8 days were achieved, which is comparable to effects seen with standard regimens of currently used anticancer drugs. These studies further establish the creatine kinase system as a promising and novel target for anticancer chemotherapy drug design.